Modern aircraft often have a considerable number of components such as flap doors, for example for covering the undercarriage when in the retracted condition, and also undercarriage assemblies themselves, in which respect there is frequently a need for such a flap door or undercarriage assembly to be firmly held in a closed or retracted position respectively, not least to ensure flight safety.
In the present specification, the term aircraft movable component is used to denote a component such as a flap door or undercarriage member, which is movable relative to a body portion of the aircraft and which is required to be suitably locked in an appropriate, generally closed or retracted, position.
Aircraft flap doors and in particular those which are used to cover the undercarriage of the aircraft when in a retracted condition, are generally relatively wide and therefore can suffer from twisting when they are caused to perform a pivotal movement, in particular because, for design reasons, the piston-cylinder unit for closing such a flap door by means of a pivotal movement thereof engages the flap door at an off-center position and indeed is usually pivotally connected to the flap door at one side or edge thereof. It can therefore frequently happen that the hydraulic piston-cylinder unit can pull the flap door into the position of closing the opening to be covered thereby, at one side of the flap door, while at the other side of the flap door which is remote from the location at which the piston-cylinder unit engages same, there is a gap which in some circumstances may even be of considerable size. As careful closing of flap doors in aircraft is an absolutely essential requirement, steps have ben taken up to the present time to ensure that, as the flap door approaches the opening to be closed thereby, the flap door actuates a valve for operating a hydraulic gripper which engages the flap door as it approaches the joining and pulls it completely into the position of securely closing the opening. The flap door is then locked in the closed position, by a separate operation. It will be appreciated that those arrangements involve a considerable level of structural expenditure which increase the cost of the aircraft and also make it heavier.